This invention relates to a system for controlling gasoline vapor emissions at a service station or stations where liquid gasoline is transferred from one container or tank to another, and more particularly to a bootless nozzle system for preventing the escape of vapors from the fuel tank of a vehicle during refueling, while at the same time preventing ingestion of fresh air into the fuel storage tank of a service station.
When a vehicle has consumed its supply of gasoline, its gasoline tank is full of gasoline vapors plus a lesser amount of liquid gasoline. During the process of dispensing a fresh supply of liquid gasoline into the tank, the vapor in the tank is displaced into the atmosphere. At the same time, fresh air is drawn down into the service station gasoline storage tank through provided vent pipes.
Gasoline vapors escaping into the atmosphere are a major source of smog and ozone. Fresh air, drawn into the storage tank, stimulates evaporation of the stored gasoline, which converts valuable gasoline into more polluting vapor.
The purpose of state of the art gasoline station vapor control systems is to solve both problems simultaneously; i.e. to prevent the escape of vapors from the vehicle tank and to prevent the ingestion of fresh air into the storage tank.
Because the volume of vapors escaping and the volume of fresh air ingested are approximately equal, the purpose of the system mechanism is to capture the vapors emitted from the vehicle tank and lead them through a conduit to the storage tank. As gasoline is dispensed from the storage tank, the storage tank ingests the vapor displaced from the vehicle tank instead of fresh air.
Pollution control agencies have increasingly mandated strict control standards for release of gasoline vapors into the atmosphere. For example, the California Air Resources Board (CARB) has mandated the following standards for vapor control systems:
1) Highest vapor efficiency in all weather conditions; PA1 2) Zero fugitive emissions (emissions of vapor through unmonitored openings or gaps in a gasoline delivery system); PA1 3) Automatic continuous self-diagnosis; PA1 4) System tolerant of leaks in service station hardware; PA1 5) System simple, tough, reliable, and economical; and PA1 6) System must use best available control technology.
One gasoline vapor recovery system well known in the art is the so-called "Balance System". Such a system consists of a tight sealing vapor recovery nozzle 1a (FIG. 2), a vapor return hose, and vapor return piping. To prevent fugitive emissions, all vent pipes are equipped with a p/v valve (pressure/vacuum valve), which will not permit venting until the tank pressure exceeds approximately +3 inches w.c.g. (water column gauge).
The "Balance System" is simple and inexpensive, but has several disadvantages. Foremost among these are its failure to meet tough control standards such as those outlined above. For example, its vapor collection efficiency is often much less than 95% (typically its efficiency runs between 60 and 95%, depending upon ambient conditions and system maintenance), which is a government mandate in many localities. This loss of efficiency is caused by the fact that gasoline vapor is very sensitive to changes in temperature; i.e. when the temperature of the vehicle tank is colder than the storage tank, vapor transferred to the storage tank will expand. This expansion causes vapor to escape through any leak or opening it can find, usually due to poor system maintenance, thus destroying the vapor collection efficiency.
The "Balance System" requires a tight vapor seal at the nozzle/vehicle interface. Typically, this seal is created by employment of a vapor collecting bellows boot 2a (FIG. 2), which is adapted to fit tightly about the vehicle tank filler neck (not shown). This type of nozzle, however, is heavy, complicated, expensive, and difficult to use. Additionally, because of the tight seal, several internal safety devices are required so as not to overpressure the vehicle tank, and to prevent recirculation of gasoline back through the nozzle and hence back to the storage tank. Also, to contain vapor, all service station components must continuously remain leaktight.
A better solution is a loose fitting nozzle bellows boot 2b in a partial seal nozzle 1b (FIG. 3) which helps collect the vapor but does not seal tightly. In such a system, in order to prevent escape of vapors around the loose fit bellows boot, the prior art teaches that it is necessary to impose a vacuum on the vapor side of the nozzle. This is done in some prior art systems, sometimes referred to as Healy systems, by placing a vapor pump in the gasoline vapor return line between the underground gasoline storage tank and the dispensing nozzle 1b. A significant disadvantage to this approach is that the gasoline vapor is pressurized on the downstream side of the vapor pump, increasing its propensity to escape through any available leak, and making compliance with environmental regulations virtually impossible.
In other prior art systems, sometimes referred to as Hasselman systems, a vapor pump is placed in a line disposed between the gasoline vapor return line and a vapor vent line which exits the underground storage tank. In this prior art approach, a vapor burner is disposed at the discharge end of the vapor vent line. The burner actuates upon the sensing of a positive pressure in the gasoline storage tank. The disadvantage of this type of prior art system is that the magnitude of the positive pressure necessary to actuate the burner is too high to prevent leakage (fugitive emissions) of the pressurized vapor, but too low to properly feed a nozzle mixing type burner.
A significant problem with all of the foregoing systems is the operator's inability to actually measure the vapor recovery efficiency of the system. For example, still another prior art system is one presently in use in Mexico, which employs a monitoring system known as the ENVIROSENTRY.TM.. This system is an electronic system which monitors the gasoline storage tank for negative or positive pressure levels. The operating theory is that if any portion of the system, such as the vent lines, vapor pumps, or nozzles, fails, typically creating a blockage in the system, a vacuum will be created in the system. The vacuum is generated because gasoline is pumped at a greater rate than vapor is collected, due to the blockage. The system is set so that when the vacuum pressure reaches -6 to -8 water column, a switch will open, cutting a signal to the control panel. The loss of signal indicates to the control panel that there is a failure and an alarm will be activated. If the condition persists for more than sixty (60) minutes, the control panel will cut current to the pumps and the service station will be shut down.
The problem with this system is that the extreme vacuum pressure of -6 to 8 water column will never be reached by the typical poorly maintained service station. At about -0.5 water column, p/v valves in the vent risers, Stage I fittings, and other components will begin to leak, permitting air into the system to reduce the negative pressure without solving the malfunction.
The ENVIROSENTRY system also theoretically operates to detect a leak of gasoline vapor in the system. The operating theory is that during normal operation some type of pressure, positive or negative, will be generated. This will vary due to climatic conditions. If the pressure is zero for a long period of time, that indicates a problem. Therefore, when the system monitor detects a zero system pressure for a specified period of time, an alarm sequence will be triggered. After a predetermined period of time of continued zero pressure, the system will cut power to the pumps and the service station will be inoperative.
Again, the problem with this approach is that, due to leaks in the system, the pressure will never remain at zero for a long period of time.
A third system condition which ENVIROSENTRY is designed to monitor is a system overpressure of greater than 2.5 inches water column. If such a condition is detected, an alarm will sound, followed by system shutdown after continued overpressure conditions for a specified period of time. Again, the problem is that leaks will activate to release vapor to the environment, lowering the system pressure before +2.5 inches water column is attained, so the system will not operate as designed. As is the case with most existing systems, it is designed to placate government regulators rather than to effectively solve real problems.
Still another prior art approach is disclosed in U.S. Pat. No. 4,680,004 to Hirt. In this patent, which is also a thermal oxidation system employing a vapor burner, it is disclosed that placement of the vapor pump at the discharge end of the vent line, just upstream of the vapor burner, is a superior approach. This arrangement, known as the "Hirt partial seal system", permits the pump to create a vacuum in all vapor spaces (the nozzle, the hose, the vapor return piping, the storage tank, and the vent line), to thereby minimize vapor escape through leaks, and producing sufficient pressure on the burner which makes a clean, sharp flame. This is a superior design to the foregoing prior art systems, but requires a moderately well sealed system including a vapor collection boot at the nozzle/vehicle interface.
The booted nozzle, as shown in FIGS. 2 and 3, has been a problem for the self-serve customer, resulting in public rejection of the entire gasoline vapor control program. Furthermore, the booted nozzles are often misused by customers, by improperly "topping off" their vehicle tanks or improperly inserting the nozzle into the vehicle fill pipe. Both of these misuses result in the escape of vapor which causes the system to fail to comply with gasoline vapor recovery regulations. This public reaction has given rise to a requirement for a bootless nozzle, as shown in FIG. 4. But the bootless nozzle has no seal at the nozzle/vehicle interface. It is obvious, therefore, that a bootless nozzle which forms no seal would be completely incompatible with the partial seal system approach taught by the Hirt U.S. Pat. No. 4,680,004.
It would be desirable, therefore, to develop a gasoline vapor recovery system which combines the vapor processing advantages of the system disclosed by the Hirt U.S. Pat. No. 4,680,004 with the customer convenience advantages of a bootless nozzle.